Altimeters are used to determine distances between an aircraft in flight and the ground or a target. A typical altimeter has a transmitter that transmits a signal at a select frequency toward the target. A gate detector is used to detect the signals reflected off of the target. Based on the knowledge of the rate of travel of the signal and the time that passed between the transmission and the receipt of the reflected signal, the distance from the aircraft to the target can be determined. In a typical altimeter, once a reflected signal is detected as a valid target, the altimeter switches from a search mode to a track mode. In the track mode a narrower gate may be used that focuses on a range around where the target was detected. In the search mode, a low pass filter (LPF) is applied to the gate detector having an absolute or square function to rectify the signal. From the LPF, a signal level can be determined. Every detector system will have an inherent noise level. Hence, a typical system will set a threshold level that is a little above the noise level that indicates when a valid target is detected. A LPF that has a low bandwidth (LBW) is typically used that reduces the fluctuations in the detected noise level and hence provides good sensitivity. Therefore, the threshold that indicates a valid signal can be set very close to the average noise level.
Jamming signals and mutual interference from other altimeters limit the efficiency of the above described altimeter system. Although the LBW LPF provides excellent sensitivity, random high energy noise spikes from jamming signals or mutual interference can cause the gate detector to indicate a target even if one is not present (a false target). The LBW LPF will tend to average a noise spike with the background noise so that the system thinks a valid weak target has been detected. In response to the false target the system will mistakenly switch from the search mode to the track mode.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an efficient and effective method of preventing the switching from a search mode to a track mode based on false target detections in an altimeter system.